Electron-optical discharge system



Aug. 21, 1956 J. c, FRANCKEN ETAL 2,760,106

ELECTRON-OPTICAL DISCHARGE SYSTEM Filed Feb. 24, 1953 2 Sheets-Sheet 2INVEN-TO'RS JAN CAREL FRANCKEN PIETER SCHAGEN AGENT nited States PatentELECTRON-OPTICAL DISCHARGE SYSTEM Jan Carel Francken and Pieter Schagen,Eindhoven, Netherlands, assignors to Hartford National Bank and TrustCompany, Hartford, Conn., as trustee Application February 24, 1953,Serial No. 338,517 Claims priority, application Netherlands March 10,1952 8 Claims. (Cl. 315-10) This invention relates to electron-opticaldischarge systems, and'more particularly, to a circuit arrangement forcontrolling the magnification of an image therein.

The use of an electron-optical system for a discharge tube, for example,the luminoscope, the image intensifier or the television camera tube,comprising an electromagnetic electron lens which consists of threemagnetic coils surrounding the tube has been proposed, in which systemeach of the coils serve a different purpose. The coil closest to thecathode of the tube usually varies the magnification of the device;however, the image on the screen becomes defocussed due to variation inthe field strength within the tube. Moreover, a second phenomena occursknown as image rotation, which always occurs in the case of magnifiedimage reproduction.

By providing a second magnetic coil, the energization of the twoseparate coils may be controlled so that the image remains sharp, if therate of magnification is changed. In addition, image rotation may bekept constant, in the case of a variable magnification, by adding athird coil, the field of which contributes to the forma tion of the lensfield. The three windings together produce magnetic fields which difierin shape and strength, but which combine to produce sharp reproductionsof the image on the photo-electric cathode on the image screen.

The winding nearest the photo-electric cathode preferably has a positionsuch that the field strength at the photo-cathode is substantially atits maximum. If only this winding is energized, a short magnetic fieldis produced and the maximum magnification is obtained. The secondwinding may be arranged adjacent the first and when it is energized, thedivergence of the magnetic field must decrease in order to obtain asmaller image. In order to maintain a sharp image, the current passingthrough the first winding must be reduced. The reduction of fieldstrength at the area of the photo-cathode and the elongation of thefield both have the effect of a smaller magnification. Consequently, themagnification may be decreased by raising the current passing throughthe second winding and by reducing the current passing through the firstwinding.

The image rotation is found to be at its minimum, if the first windingis only energized and the magnification is at its maximum. The smallerthe magnification, the greater the image rotation. The variation in theimage rotation may be neutralized by extending the magnetic field to acertain extent in the direction of the image screen. For this purpose athird winding may be provided between the two aforesaid windings and theimage screen. The field of this winding must primarily be operative inthe area free from electric fields. It compensates for the variation inthe image rotation without seriously aifecting the magnification and thesharpness of the image, since the winding, which is remote from thecathode, exerts little influence on the field in the proximity of thecathode.

The main object of the invention is to provide a simple 2,760,106Patented Aug. 21, 1956 arrangement for continuously varying themagnification of the image while retaining a sharp image and minimizingany image rotation. According to the invention, it has been found that asimple arrangement for continuously varying the magnification of theimage in an electron-optical image system comprising three coil windingscan be provided by maintaining substantially constant the sum of theenergizing currents passing through the three coil windings throughoutthe desired control range of the system.

The invention will now be described in greater detail with reference tothe accompanying drawing in which:

Figs. 1 and 2 show, respectively, the variation of ampere-turns andcurrent with magnification of a three winding electron-optical system;

Figs. 3 and 4 are circuit diagrams of two arrangements in accordancewith the invention for maintaining a substantially constant sum of allthree energizing currents;

Fig. 5 shows one form of construction in which the invention may beemployed.

With a definite geometrical arrangement of the energizing windings, thenumber of required ampere-turns for each specific magnification isdetermined. The currents required, however, will be determined by thechoice of the number of turns for each winding. The requiredampere-turns for the various windings may be designated by A1(V), A2(V)and Aa(V), respectively. Each of them is a function of the magnificationV and may be plotted in a graph, as shown in Fig. 1. In addition to therequired number of ampere-turns for each separate winding, the variationof the total number of ampereturns, required for continuouslycontrollable magnification and constant image rotation and sharpness isalso indicated in Fig. 1.

If the three coils have n1, I12 and n3 turns, respectively, the sum ofthe currents through the three coils is:

In general, 1' will also be a function of V. The values of n1, In and113 may now be chosen such that i is constant for all values of V in thedesired control range in accordance with the invention in the followingmanner. By choosing m, the value i1 is determined at maximummagnification, and hence also 1', since in this case is and is are zero(see Fig. l). The value of ma follows from Fig. 1 for another value ofthe magnification at which the third is not energized (i3=0). Finally,the value of 213 may be determined at a magnification at which all threewindings are energized. In Fig. 2, i1, 1'2, i3 and i are indicated asfunctions of V. It is found that between V=3.75 and V=7.75, i isconstant with a tolerance of about 1% and the objects of the inventionare attained.

Figs. 3 and 4 show circuit-arrangements which may be used in a deviceaccording to the invention and which carry out the foregoing method.Referring to Fig. 3, to the terminals 1 and 2 of a direct-current source(not shown) are connected two stabilizing tubes 3 and 4, e. g. glowdischarge tubes, which, in cooperation with the resistor 5, maintain thevoltage between two connecting terminals 6 and 7 constant. The voltageat these points is supplied to the series combination of three systemsa, b and c. The system a comprises a resistor 8, with which a firstwinding 9 connected in series with an adjusting resistor 10 is connectedin parallel. The winding 9 usually surrounds the photo-electric cathodeof the device.

The system b comprises a potentiometer 11, with which a second winding12 in series with an adjusting resistor 13 is connected in parallel;this winding 12 serves to control the magnification.

The system c, which is constructed in a similar manner to that of b,comprises a potentiometer 14, with which a third winding 15 is connectedin parallel; this winding serves mainly for correcting the imagerotation. With this winding 15 is also connected in series an adjustingresistor 16.

It is evident from Fig. 2 that at the maximum magnification the currentpassing through the second and third windings 12 and 15 is zero. Thecurrent strength in the winding 9 may then be adjusted with the aid ofthe adjusting resistor 10 to the correct value, which may be derivedfrom Fig. 2. Moreover, the resistors of the winding-included branches ofthe systems b and c are adjusted with the aid of the resistors 13 and 16to the same value R as that of the winding-included branch of the systema. Consequently, i1R+i2R+i3R=iR=v. The voltage v is kept constant, andhence i is also constant.

In the control-range between point 17 and 18 of the potentiometer 11,the current through the winding remains adjusted to zero value. In thisrange no correction of the image rotation is required. The currentthrough the winding 12 increases and the current through the winding 9decreases to the same extent, so that their sum remains constant.

In the further control-range, provisions must be made so that the sum ofthe currents through the windings 12 and 15 is equal to the diiferencebetween the constant current i and the current i1 required forenergizing the winding 9. At the beginning of the control-range of thecurrent through the winding 15, the resistance of the potentiometer isstill very low, and at the end of the controlrange the resistance of thepotentiometer branch must have a value at which a current is (see Fig.2) passes through the winding 15, whereas the current through thewinding 12 corresponds to a value i2. For the adjust ment of theintermediate values of is, a linear resistance distribution may be used.Consequently, only the potentiometer 11 need have a special resistancevariation, the value of each adjustment being found when the sum of thecurrents through the windings 12 and 15 have the required value, andtherefore, instead of three controlresistors, only two are required, oneof which may, moreover, be a linear resistor. If 'a slightly lessaccurate control than in the case of complete adaptation of the thirdpotentiometer to the theoretically correct resistance distributionsuffices, the third resistor may be composed of a number of linearparts, thereby yielding a material simplification.

Referring to Fig. 4, which shows a further modification according to theinvention, the windings 9, 12 and 15 are connected in parallel and inseries with a current controlswitch 20 to the terminals 1 and 2 of acurrent source. The current control-switch 20 is such that it keeps thecurrent at a constant value; for this purpose use may be made, forexample, of an iron wire resistor in a gaseous atmosphere.

By the choice of n1 the current i is determined as explained beforehand.For reasons to be explained hereinafter, it is desirable to provide avalue of the resistance of the current branch of the circuit includingthe winding 9 such that the voltage thereacross is at least 100 v. atthe lowest current. For this purpose, the resistor 21 may be connectedin series with the winding 9.

A discharge tube 22 comprising a control-grid serves to control thecurrent strength through the winding 12, since it is not quite possibleto cause the current to increase regularly from zero with the aid of aseries resistor. For this purpose a discharge tube is very suitable andif, moreover, use is made of a pentode tube, the control of the currentis substantially independent of the voltage, if the latter does not dropbelow a minimum value of, for example, 100 v.

The current across the winding 15 must be controllable from zero; forthis purpose use is also made of a discharge tube 23 comprising acontrol-grid. The control-voltages for the tubes 22, 23 are derived froma pair of potentiometers 24 and 25, which are included in series withthe wind- 4 ings in the current supply leads on the cathode side of thedischarge tubes 22, 23.

After the choice of the number of turns for each winding is made asdescribed above, the current strength associated with a number of valuesof the magnification are determined, as well as the adjustments of thepotentiometers at which these current strengths are obtained. As in thedevice shown in Fig. 3, one of the otentiometers may be a linearresistor and the other may be of particular construction such that eachadjustment is associated with a resistance value which controls thecurrent through the associated winding to the difference between thetotal current and the sum of the currents through the two otherwindings, in accordance with the prescribed value and the invention.

The broken connecting line 26 in Figs. 3 and 4 indicates that thesliding contacts of the resistors 11 and 14 and 24 and 25, respectively,are displaced simultaneously with the aid of 'a common control-knob 2'7.

Fig. 5 shows a cross-sectional view of an image iconoscope type oftelevision camera tube to illustrate the general combination of theinvention. The camera tube comprises an envelope 32 containing aphotocathode 33 at one end and a target electrode 34 at the other end.Electron beam producing means 35 are located in a side branch of theenvelope 32 for scanning the target electrode 34. The image to betelevised is optically projected onto the photcathode 33 producing acorresponding electron image, which is then electron-optically projectedonto the target 34. The latter is accomplished by a three winding systememploying a first winding 9 surrounding the photocathode and a secondwinding 12, which latter windings serve to control the magnification. Athird winding 15 serves mainly to control the image rotation. Aconductive layer 36 is also present in the tube to establish an electricfield at the photocathode 33. The energizing means shown schematicallyrepresents the circuits illustrated in Figs. 3 and 4.

While we have described our invention in connection with specificembodiments and applications, other modifications thereof will bereadily apparent to those skilled in this art without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In combination, a discharge device, an electronoptical systemassociated with said discharge device including three coil windings, andmeans for energizing said windings; said energizing means comprising asource of current, and means to vary the current passing through atleast one of said windings and to simultaneously maintain substantiallyconstant the sum of the currents passing through said three windingsthroughout a given range of adjustment.

2. In combination, a discharge device including a photo-cathode, anelectron-optical system associated with said discharge device andcomprising three successivelyarranged coil windings surrounding saiddevice, and means for energizing said windings; said energizing meanscomprising a source of current, means for concurrently varying thecurrent passing through at least two of said windings to change themagnification of the electron-optical system over a predetermined range,and means for maintaining substantially constant the total currentpassing through all three windings throughout said predetermined rangeto maintain a focussed image and minimize any image rotation.

3. The combination set forth in claim 2, wherein said last-named meansincludes constant current control means arranged in series with thethree windings.

4. The combination set forth in claim 2, wherein a fixed resistor isassociated with one of the windings, and mechanically-coupled variableresistors are associated each with one of the other two windings.

5. A circuit-arrangement for energizing an electromagnetic-lens systemcomprising three coil windings for an electron-optical discharge system,comprising a source of current having a substantially constant voltage,said three windings being connected in series with said source, aresistor connected in parallel with one of said windings, a pair ofpotentiometers each connected in parallel with one of the other twowindings, and means to concurrently vary the resistance of saidotentiometers and to simultaneously maintain the sum of the currentspassing through said three windings substantially constant throughout agiven range of adjustment.

6. A circuit arrangement as claimed in claim 5, in which at least one ofthe potentiometers is a linear resistor.

7. A circuit-arrangement for energizing an electromagnetic-lens systemcomprising three coil windings for an electron-optical discharge system,comprising a source of substantially constant current, said threewindings being connected in parallel with said source, a pair of variable resistive elements each connected in series with one of saidwindings, and means to concurrently vary the resistance of said variableresistive elements and to simultaneously maintain the sum of thecurrents passing 6 through said three windings substantially constantthroughout a given range of adjustment.

8. A circuit-arrangement as claimed in claim 7, in which the variableresistive elements are constituted by electric discharge tubescomprising a control grid, and a pair of potentiometers are provided inseries with the parallel-interconnected windings, the control voltagesfor said control grids of said electric discharge tubes being derivedfrom said otentiometers.

References Cited in the file of this patent UNITED STATES PATENTS2,007,380 Morlock July 9, 1935 2,228,821 Hansen Jan. 14, 1941 2,240,700Kemp May 6, 1941 2,447,804 Holst Aug. 24, 1948 2,454,378 Forgue Nov. 23,1948 2,552,357 Wendt May 8, 1951 2,587,420 Wendt Feb. 26, 1952 2,654,854Seright Oct. 6, 1953

